1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus for generating ultraviolet light by applying a laser beam to a target material to turn the target material into plasma.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, micro-fabrication at 60 nm to 45 nm, further, micro-fabrication at 32 nm and beyond will be required. Accordingly, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there is an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”). The LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than several tens of watts to one hundred of watts.
In the LPP type EUV light source apparatus, by injecting a target material from a nozzle and applying a laser beam to the target material, the target material is excited and turned into plasma. Various wavelength components including extreme ultraviolet (EUV) light are radiated from the plasma. Then, a desired wavelength component of them is selectively reflected and collected by using a collector mirror (an EUV collector mirror), and outputted to a unit using EUV light (e.g., exposure unit). For example, in order to collect EUV light having a wavelength near 13.5 nm, an EUV collector mirror having a reflecting surface, on which a multilayer coating of alternately stacked molybdenum and silicon (Mo/Si multilayer coating) is formed, is used.
In the LPP type EUV light source apparatus, the influence of neutral particles and ions having various velocities emitted from plasma on the EUV collector mirror is problematic. Since the EUV collector mirror is located near the plasma, the neutral particles and low-velocity ions emitted from the plasma adhere to the reflecting surface of the EUV collector mirror and reduce the reflectance of the EUV collector mirror. On the other hand, the fast ions emitted from the plasma damage the multilayer coating formed on the reflecting surface of the EUV collector mirror (in this application, this is referred to as “sputtering”).
It is considered that neutral particles can be suppressed by optimizing the process of generating fully-ionized plasma according to various methods such as a double-pulse application method and a minimum mass target method that is described in International Publication WO 02/46839 A2. However, ion generation is inevitable as long as the plasma is generated. Accordingly, measures for ions are absolutely necessary.
The low-velocity ions adhere to the EUV collector mirror and reduce the reflectance thereof. Since the ions only adhere to the EUV collector mirror, in principle, the adhesions can be removed by a cleaning technology using a reactive gas or the like. After cleaning, the reflectance of the EUV collector mirror is recovered and the EUV collector mirror can continuously be used. However, in order to fulfill the requirement for an EUV light source apparatus for exposure (a period in which the reflectance decreases by 10% is one year or more), an amount of adherence (thickness) of a metal film on the reflecting surface of the EUV collector mirror is acceptable as a very small value of about 0.75 nm for tin (Sn). Accordingly, it is necessary to perform high-speed cleaning at a high frequency.
On the other hand, fast ions sputter the surface of the EUV collector mirror, and damage the reflecting coating to reduce the reflectance. When the EUV collector mirror is damaged and its reflectance becomes lower, replacement of the EUV collector mirror is required. A technology of reproducing the reflecting coating within the EUV light source apparatus is also available, however, it is necessary to add a high-accuracy coating formation apparatus for providing high surface flatness of about 0.2 nm (rms), for example, and that increases cost. Further, due to the damage distribution, it is substantially impossible to obtain a uniform reflectance distribution even when the reflecting coating is reproduced.
Therefore, generally, several hundreds of layers of reflecting coatings have been deposited for extending the lifetime of the EUV collector mirror until replacement. Further, as a method of reducing the damage density of fast ions, there is a method of separating the distance between the EUV collector mirror and a plasma generation point (light emission point). In this case, there has been a problem that the catching solid angle of the EUV light becomes smaller and the output of available EUV light becomes lower. In order to solve the problem, for example, a method of using an EUV collector mirror having a large diameter equal to or more than φ500 mm is conceivable. However, there are problems that it is difficult to generate several hundreds of layers of reflecting coatings while maintaining the surface roughness and form accuracy, and such an EUV collector mirror is expensive even if it can be fabricated.
In order to solve the problems, Japanese Patent Application Publication JP-P2005-197456A discloses an EUV light source apparatus including a magnetic field generating unit for generating a magnetic field within a collective optics when current is supplied, and trapping charged particles emitted from plasma by using the magnetic field to prevent adherence of the target material to the EUV collector mirror and sputtering of the EUV collector mirror.
FIG. 21 schematically shows a configuration of the EUV light source apparatus according to JP-P2005-197456A. The EUV light source apparatus includes a target supply unit, a driver laser for applying a laser beam to a target, and an EUV collector mirror for collecting EUV light to output the EUV light. As shown in FIG. 22, a pair of electromagnetic coils having magnetic poles directed toward the same direction are provided with a part, where the laser beam is applied to the target, in between. The pair of electromagnetic coils form a mirror magnetic field around the laser application part and capture the charged particles flying from the target within the magnetic field to prevent the charged particles from reaching the EUV collector mirror.
However, in order to deflect fast ions having energy up to 10 keV not to reach the EUV collector mirror, a strong magnetic field is necessary. In order to form a strong magnetic field in a space around the EUV collector mirror as shown in FIG. 21, Helmholtz coils having a gap equal to or more than the diameter (e.g., φ300 mm) of the EUV collector mirror should be prepared. Such electromagnetic coils are very large and not only cause constraints on design but also cause upsizing of the apparatus and increase in the apparatus cost.
Further, since a strong magnetic field is generated within and around the EUV light source apparatus, materials that can be used inside and outside of the EUV apparatus are limited. This is because it should be avoided that the magnetic field acts on the structure or the servo motor and causes deformation of the structure or malfunction of the motor. Furthermore, there are problems of generating secondary cost in such a case where it is necessary to provide a magnetic field shield to cover the EUV light source apparatus and prevent malfunction of other apparatuses due to the strong magnetic field.